In recent years, the mining conditions of oil wells and gas wells (hereinafter, oil and gas wells are collectively referred to simply as “oil wells”) of crude oil, natural gas, and the like have become severer. Concerning the mining environments, as the mining depth increases, the atmosphere contains CO2, H2S, Cl−, and the like, and the mined crude oil and natural gas also contain large amounts of H2S. Therefore, the requirement on performance of a line pipe for transmitting the crude oil and natural gas has also become more rigid, and a demand for steel pipes for line pipe having sulfide resistance has increased.
The standards of National Association of Corrosion Engineers (NACE) specify the highest hardness of steel for steels used in H2S environments from the viewpoint of sulfide stress cracking resistance (hereinafter, referred also to as “SSC resistance”), which is 250 HV10 or lower for carbon steels. Also, to assure safety, steels may, in some cases, be required to have the highest hardness of 230 HV10 or lower in consideration of the hardening of weld heat affected zone. Therefore, for steels that are required to have sulfide resistance, the improvement in technique for restraining hardness has become an important problem. The “HV10” means a “hardness symbol” in the case where Vickers hardness test is conducted with the test force being 98.07 N (10 kgf).
In the case where a high-strength seamless steel pipe for line pipe is produced, unlike the production process of UO steel pipes in which controlled rolling is performed, it is a common practice to perform quenching followed by tempering, in order to assure strength. For a low-alloy steel such as a steel for line pipe, by the ordinary quenching tempering treatment, martensite is not formed, and a micro-structure consists mainly of bainite. Because of great dependence on cooling rate, the micro-structure sometimes differs between the surface and the interior of steel pipe. Therefore, as compared with the interior in which the cooling rate is low, the surface, on which the cooling rate is high, tends to have a high hardness. As the result, the highest hardness on the surface becomes high with respect to the strength of steel. This tendency becomes remarkable as the strength increases and the wall thickness increases because of the increase in amount of added alloying elements.
Patent Document 1 discloses a seamless steel pipe having a wall thickness of 30 mm or larger and a high strength of X65 grade or higher (yield strength: 448 MPa or higher). Also, Patent Document 2 discloses a seamless steel pipe having a strength of X70 grade or higher and excellent hydrogen-induced cracking resistance.
To solve the problem of increase in the highest hardness, Patent Document 3 proposes a method for softening only the casehardened layer by means of local heating such as induction heating due to high-frequency current. Also, Patent Document 4 proposes a method for performing the quenching tempering treatment from a two-phase region of austenite and ferrite. Further, Patent Document 5 proposes a method in which cooling is stopped by controlled cooling in mid-course and the outer near-surface portion is subjected to heat recuperation by the heat in the high-temperature portion on the surface side in a steel pipe, before cooling again the outer near-surface portion.